Formic Acid as a Hydrogen Donor for Catalytic Transformations of Tar

: Speciﬁc features of the catalytic tar cracking in the presence of formic acid, BEA zeolite and 8% Ni-2.5% Mo / Sibunit catalyst were studied at 350 ◦ C and 1.0 MPa pressure. The obtained results evidenced that formic acid can be used as a hydrogen donor during catalytic reactions. The formic acid addition made it possible to perform e ﬃ cient hydrocracking of heavy feed such as tar. It was found that both the tar conversion and selectivity to light (gasoline-diesel) fractions grew in the sequence: tar < (tar - formic acid) < (tar - formic acid - BEA zeolite) < (tar - formic acid - BEA zeolite - 8% Ni-2.5% Mo / Sibunit catalyst). Furthermore, signiﬁcantly lower concentrations of impurities containing sulfur and nitrogen were observed for the (tar - formic acid - BEA zeolite - 8% Ni-2.5% Mo / Sibunit catalyst) system. For example, the sulfur and nitrogen concentrations in the tar precursor were 1.50% and 0.86%, respectively. Meanwhile, their concentrations in the liquid products after the catalytic cracking were 0.73% and 0.18%, respectively.


Introduction
Due to increasing worldwide demand for motor fuel, the need to utilize non-traditional heavy oil feedstock (HOF), including heavy oils, natural bitumen, heavy residual oil fractions (tar, black oil), bituminous sands and pyroshales is also rapidly growing. A number of methods for deep processing of heavy oil fractions, heavy oils and oil residues in the presence of catalysts have been reported. Traditional approaches to HOF processing can be divided into two main types [1][2][3][4]. The first type includes deasphalting, thermal processes: gasification and carbonization (delayed, flexicoking, etc.), viscosity breaking and catalytic cracking. The second type includes various hydrogenation processes. Therefore, there is great interest in hydrocracking of heavy residual oil fractions, particularly tar, aimed at the production of the main products of the gasoline and diesel fractions that are in great demand. The chemistry of hydrogenation processes is based on the efficient use of hydrogen. However, reactors used for hydrocracking processes operate under high pressure and require complex instrumentation. Thus, researchers are currently looking for new sources of hydrogen.
One of the actively developing areas combining the advantages of the thermal and hydrogenation refinement processes is catalytic vapor cracking of HOF [5][6][7]. In this process water acts as a hydrogen donor. Its participation makes it possible to increase the yield of the light fraction (boiling point below 360 • C) and decrease the coke yield as well as the contents of sulfur and heteroatoms in the liquid products. The use of catalysts based on Ni [8,9], Fe [10,11], Mo [8,12], Co [13] and Zr [14,15] in the catalytic vapor cracking favors stronger HOF interaction with water, including partial oxidation, low-temperature partial steam reforming and catalytic cracking processes, thus substantially improving the overall efficiency of the process.
A different approach was demonstrated in other publications. It was suggested to add methane to CO 2 for cracking of resins obtained by coal pyrolysis. Due to its high H/C ratio, methane is considered to be a hydrogen substitute. Hydrogen is formed by methane reforming with carbon dioxide [16,17].
However, these approaches are complicated for practical implementation because two complex processes have to be optimized: hydrogen production and the hydrocracking itself. The use of formic acid as a hydrogen donor significantly simplifies the process. Formic acid is a high-quality organic hydrogen source, as it has relatively high hydrogen content (4.4 wt.%), low flammability and low toxicity. It is important to note that formic acid can be prepared from the biomass or from CO 2 [18][19][20].
Delayed carbonization is the most popular scheme for processing of tar and other heavy residues. In the current study, modification of the tar carbonization was attempted. The formic acid addition was suggested as a means to regulate the yield and properties of the formed carbonization products.
The goal of the current study was to investigate the effect of the formic acid addition and different catalysts on the yield and properties of liquid tar hydrocracking products.

Materials and Methods
Tar from the Omsk Oil Refinery (Omsk, Russia) was used as a feedstock. The elemental and fraction compositions of the used tar are reported in Table 1. Hydrocracking catalysts are bifunctional. Their hydrogenation-dehydrogenation function and activity in all reactions related to hydrorefining is usually associated with MoS 2 or WS 2 promoted with nickel sulfide. Amorphous or crystalline aluminosilicates, Y [21] or BEA [22][23][24] zeolites are typically responsible for the cracking function. Based on this information, a nickel-molybdenum catalyst and a BEA zeolite were used in this study for the tar hydrocracking.
The use of carbon supports for tar carbonization is desirable because they do not increase the ash content of the resulting oil coke. The artificial mesoporous carbon support "Sibunit" [25,26], which has a surface of 480 m 2 /g and a graphite-like structure, was used for the catalyst synthesis. A TEM image of the Sibunit support is shown in Figure 1. The support particles have an egg-shell shape. Sibunit is a graphite-like carbon material with an interlayer distance of d 002 = 3.52 Å and an average crystallite size in this direction of about 25 Å.
The 8% Ni-2.5% Mo/Sibunit catalyst was prepared by impregnation. Precalculated amounts of Ni(NO 3 ) 2 ·6H 2 O and (NH 4 ) 2 MoO 4 (Reakhim, 98% purity) were dissolved in 10 mL of distilled water and added to a beaker containing 10 g of Sibunit. Then, the sample was dried on a hot plate with a magnetic stirrer and calcined in a muffle furnace in air at 250 • C for 30 min. The obtained NiO-MoO/Sibunit sample was reduced in the hydrogen flow in a flow reactor at 400 • C for 1 h. After reduction the catalyst composition was 8% Ni-2.5% Mo/Sibunit. The XRD pattern of 8% Ni-2.5% Mo/Sibunit reduced in hydrogen is shown in Figure 2. The 8% Ni-2.5% Mo/Sibunit catalyst was prepared by impregnation. Precalculated amounts of Ni(NO3)2·6H2O and (NH4)2MoO4 (Reakhim, 98% purity) were dissolved in 10 ml of distilled water and added to a beaker containing 10 g of Sibunit. Then, the sample was dried on a hot plate with a magnetic stirrer and calcined in a muffle furnace in air at 250 °C for 30 min. The obtained NiO-MoO/Sibunit sample was reduced in the hydrogen flow in a flow reactor at 400 °C for 1 h. After reduction the catalyst composition was 8% Ni-2.5% Mo/Sibunit. The XRD pattern of 8% Ni-2.5% Mo/Sibunit reduced in hydrogen is shown in Figure 2.  The peaks observed in the XRD pattern of 8% Ni-2.5% Mo/Sibunit can be attributed to nickel metal or a nickel-molybdenum phase ( Figure 2). The peak at 2θ ~ 44° is formed by overlapping of different peaks and has a shoulder. This shoulder (2θ ~ 42-43°) can be attributed either to graphite or to a nickel-molybdenum phase. The position of the peak close to the shoulder (2θ ~ 44.3°) is typical for the nickel metal phase. The diffraction characteristics of the active component of the Ni-Mo/Sibunit catalyst are reported in Table 2.   The 8% Ni-2.5% Mo/Sibunit catalyst was prepared by impregnation. Precalculated amounts of Ni(NO3)2·6H2O and (NH4)2MoO4 (Reakhim, 98% purity) were dissolved in 10 ml of distilled water and added to a beaker containing 10 g of Sibunit. Then, the sample was dried on a hot plate with a magnetic stirrer and calcined in a muffle furnace in air at 250 °C for 30 min. The obtained NiO-MoO/Sibunit sample was reduced in the hydrogen flow in a flow reactor at 400 °C for 1 h. After reduction the catalyst composition was 8% Ni-2.5% Mo/Sibunit. The XRD pattern of 8% Ni-2.5% Mo/Sibunit reduced in hydrogen is shown in Figure 2.  The peaks observed in the XRD pattern of 8% Ni-2.5% Mo/Sibunit can be attributed to nickel metal or a nickel-molybdenum phase ( Figure 2). The peak at 2θ ~ 44° is formed by overlapping of different peaks and has a shoulder. This shoulder (2θ ~ 42-43°) can be attributed either to graphite or to a nickel-molybdenum phase. The position of the peak close to the shoulder (2θ ~ 44.3°) is typical for the nickel metal phase. The diffraction characteristics of the active component of the Ni-Mo/Sibunit catalyst are reported in Table 2.  The peaks observed in the XRD pattern of 8% Ni-2.5% Mo/Sibunit can be attributed to nickel metal or a nickel-molybdenum phase ( Figure 2). The peak at 2θ~44 • is formed by overlapping of different peaks and has a shoulder. This shoulder (2θ~42-43 • ) can be attributed either to graphite or to a nickel-molybdenum phase. The position of the peak close to the shoulder (2θ~44.3 • ) is typical for the nickel metal phase. The diffraction characteristics of the active component of the Ni-Mo/Sibunit catalyst are reported in Table 2.  Tables 3 and 4.  Catalytic reactions of tar were performed in an autoclave at 350-500 • C and 1 MPa pressure. A scheme of the autoclave unit for catalytic reactions of tar is shown in Figure 3. A tar sample (~16 g) or a tar sample (~16 g) with a catalyst (0.4 g) was loaded in the sample holder basket. In another series of experiments, a tar sample (~16 g) with formic acid (0.4 g) or a tar sample (~16 g) with formic acid (9 g) and a catalyst (0.4 g) was loaded in the sample holder basket. Then, the autoclave was heated to the desired temperature and maintained at it for 2 h.
Energies 2020, 13, x FOR PEER REVIEW 4 of 12 BEA zeolite (SiO2/Al2O3 = 27) produced by AZKiOS (Angarsk, Russia) was used. Its characteristics are reported in Tables 3 and 4. Crystallinity, % 86 5 SiO2/Al2O3 ratio 27 Catalytic reactions of tar were performed in an autoclave at 350-500 °C and 1 MPa pressure. A scheme of the autoclave unit for catalytic reactions of tar is shown in Figure 3. A tar sample (~16 g) or a tar sample (~16 g) with a catalyst (0.4 g) was loaded in the sample holder basket. In another series of experiments, a tar sample (~16 g) with formic acid (0.4 g) or a tar sample (~16 g) with formic acid (9 g) and a catalyst (0.4 g) was loaded in the sample holder basket. Then, the autoclave was heated to the desired temperature and maintained at it for 2 h. The gas phase components were analyzed using a Kristall-2000M gas chromatograph (Chromatec, Yoshkar-Ola, Russia). A thermal conductivity detector was used for analysis of H2S, COS, SO2, H2, O2, CO2 and CH4 after their separation on a packed column (2 mm × 1.5 m) filled with SKT activated carbon "AQUACHEM" Kazan. A flame ionization detector was used for analysis of the gas phase organic components after separation on a packed column (2 mm × 3 m) filled with Hayesep Q + 0.9% PTMSP (Hayes Separations, Inc. of Bandera, TX, USA).
The concentrations of sulfur and nitrogen and the H:C ratio in the initial tar and products of its processing were determined using a VARIO EL CUBE CHNS-O-analyzer (Elementar Analysensysteme GmbH, Langenselbold, Germany). The fraction compositions of the tar and products formed from it were determined by imitation distillation according to ASTM D7169 using an Agilent 7890В gas chromatograph (Santa Clara, CA, USA). The gas phase components were analyzed using a Kristall-2000M gas chromatograph (Chromatec, Yoshkar-Ola, Russia). A thermal conductivity detector was used for analysis of H 2 S, COS, SO 2 , H 2 , O 2 , CO 2 and CH 4 after their separation on a packed column (2 mm × 1.5 m) filled with SKT activated carbon "AQUACHEM" Kazan. A flame ionization detector was used for analysis of the gas phase organic components after separation on a packed column (2 mm × 3 m) filled with Hayesep Q + 0.9% PTMSP (Hayes Separations, Inc. of Bandera, TX, USA).
The concentrations of sulfur and nitrogen and the H:C ratio in the initial tar and products of its processing were determined using a VARIO EL CUBE CHNS-O-analyzer (Elementar Analysensysteme GmbH, Langenselbold, Germany). The fraction compositions of the tar and products formed from it were determined by imitation distillation according to ASTM D7169 using an Agilent 7890B gas chromatograph (Santa Clara, CA, USA).
The elemental compositions of solid phases were determined using the X-ray fluorescence spectrometer ARL Perform'X (Bruker, Germany) with a Rh anode for fluorescence excitation. Powder samples were ground in an agate mortar to a homogeneous finely dispersed state. Powder and liquid (viscous) samples were placed into a special holder covered with a Spectrolenesix polypropylene film that is transparent to the X-ray irradiation. Then, this holder was placed into the spectrometer chamber filled with helium. Automatic spectrum smoothing, background subtraction and calculation of the concentrations were performed using the UniQuant software package (Germany).
The phase composition of the samples was studied by the X-ray diffraction analysis. Diffraction patterns were recorded with a Thermo ARL X'TRA diffractometer (Thermo Fisher Scientific, Basel, Switzerland) using CuKα radiation with a wavelength of 1.54184 Å. The 2θ scan range was 5-75 • , the scanning step was 0.05 • and the accumulation time at each point was 5 s. Diffraction (Powder Diffraction Files) (PDF) and structural (Inorganic Crystal Structure Database) (ICSD) databases were used to identify the phase compositions. The average crystallite sizes were determined using the Selyakov-Scherrer formula from the integral widths of the diffraction peaks.
High-resolution transmission electron microscopy (HRTEM) images were obtained using a JEOL JEM-2010 (Tokyo, Japan) electron microscope with a lattice resolution of 0.14 nm.
The experiments on the formic acid (FA) decomposition were performed in a flow installation using a quartz reactor (inner diameter 6 mm). The catalyst loading was 20 mg. The catalyst was uniformly mixed with 0.5 cm 3 of quartz. The feed consisted of 5 vol.% FA in helium. Its flow rate was 20 cm 3 /min. The experiments were performed in a temperature-programmed mode with a temperature increase rate of 2 • C/min. The reaction progress was followed by monitoring the release of CO and CO 2 .

Thermal Transformations of Tar
The tar carbonization was studied in the temperature range of 350-500 • C. At 350 • C, the tar carbonization yielded gaseous and liquid products as well as oil coke. The amount of liquid product was about 15 wt.%. At 450 • C only gaseous and solid carbonization products were formed. The temperature increase favors conversion of liquid products into gases. The composition of gaseous products obtained at 350-500 • C is reported in Table 5. Methane, ethane and propane were the main gaseous carbonization products. The methane concentration in the products grew with carbonization time. Significant amounts of H 2 S and COS were observed in the reaction products. The contents of the sulfur-containing compounds in gaseous products formed during the tar carbonization at 450 • C are reported in Table 6. Hydrocarbon gases, as well as sulfur-containing H 2 S and COS products, were formed during the process. Gaseous products were periodically removed from the autoclave, and therefore a release of weakly bound sulfur from the tar decreased as the coking process proceeded. This explains the decreasing concentration of sulfur-containing products in the gas as the reaction time increased. Table 6. Concentrations of sulfur-containing compounds in gaseous products of tar carbonization at 450 • C as a function of reaction time.

Catalytic Decomposition of Formic Acid
Hydrogen can be produced from formic acid by a catalytic or non-catalytic reaction. The formic acid decomposition in the gas phase can follow two pathways: dehydrogenation with the formation of H 2 and CО 2 (Equation (1)), and dehydration with the formation of CОand H 2 O (Equation (2)). (1) Formic acid decomposition can be catalyzed by a number of metals. Palladium catalysts are the most efficient for the hydrogen production from formic acid according to Equation (1) [27][28][29][30][31]. Nickel catalysts have lower activity [32]. However, from the economic viewpoint nickel catalysts deserve more attention than noble metal catalysts. The catalyst supports play an important role in the catalytic activity, especially when a highly dispersed active metal is used. Porous carbon materials are suitable candidates, since they have all the properties required for such support.
Experiments in a flow installation with a quartz reactor were carried out to determine the pathways of the formic acid decomposition over the 8% Ni-2.5% Mo/Sibunit catalyst. The 8% Ni-2.5% Mo/Sibunit catalyst was pre-treated in a flow of 5 vol.% FA in helium at 300 • C to remove the oxide layers formed by contact of metal particles with air. The FA conversion was calculated as a ratio of the sum of the CO and CO 2 concentrations to the initial FA concentration. The selectivity to CO 2 (H 2 ) was determined as a ratio of the CO 2 concentration to the sum of the CO and CO 2 concentrations. The results of the formic acid catalytic decomposition over the 8% Ni-2.5% Mo/Sibunit catalyst are presented in Figure 4.
Energies 2020, 13, x FOR PEER REVIEW 6 of 12 reported in Table 6. Hydrocarbon gases, as well as sulfur-containing H2S and COS products, were formed during the process. Gaseous products were periodically removed from the autoclave, and therefore a release of weakly bound sulfur from the tar decreased as the coking process proceeded. This explains the decreasing concentration of sulfur-containing products in the gas as the reaction time increased.

Catalytic Decomposition of Formic Acid
Hydrogen can be produced from formic acid by a catalytic or non-catalytic reaction. The formic acid decomposition in the gas phase can follow two pathways: dehydrogenation with the formation of Н2 and СО2 (Equation (1)), and dehydration with the formation of СО and H2O (Equation (2)). Formic acid decomposition can be catalyzed by a number of metals. Palladium catalysts are the most efficient for the hydrogen production from formic acid according to Equation (1) [27][28][29][30][31]. Nickel catalysts have lower activity [32]. However, from the economic viewpoint nickel catalysts deserve more attention than noble metal catalysts. The catalyst supports play an important role in the catalytic activity, especially when a highly dispersed active metal is used. Porous carbon materials are suitable candidates, since they have all the properties required for such support.

HCOOH (gas
Experiments in a flow installation with a quartz reactor were carried out to determine the pathways of the formic acid decomposition over the 8% Ni-2.5% Mo/Sibunit catalyst. The 8% Ni-2.5% Mo/Sibunit catalyst was pre-treated in a flow of 5 vol.% FA in helium at 300 °C to remove the oxide layers formed by contact of metal particles with air. The FA conversion was calculated as a ratio of the sum of the CO and CO2 concentrations to the initial FA concentration. The selectivity to CO2 (H2) was determined as a ratio of the CO2 concentration to the sum of the CO and CO2 concentrations. The results of the formic acid catalytic decomposition over the 8% Ni-2.5% Mo/Sibunit catalyst are presented in Figure 4. The formic acid decomposition on the 8% Ni-2.5% Mo/Sibunit catalyst preferentially follows the pathway of Equation (1) with the formation of CO 2 and H 2 (Figure 4). Similar experiments on the formic acid decomposition were performed in an empty autoclave without a catalyst. About 3 mL of formic acid was placed into a 300 mL autoclave. The autoclave was gradually heated to 250 • C and maintained at this temperature for 30 min. Then, the temperature was increased in several steps to 350 • C. It was found that the pressure growth in the autoclave began at 120 • C. At 250 • C the pressure was equal to 4 atm. The gas phase composition was studied by gas chromatography. The dependence of the gas phase composition and pressure on temperature is reported in Table 7. Even in the empty autoclave the formic acid decomposition followed the pathway of Equation (1). This is most likely caused by the catalytic effect of the steel autoclave walls on the formic acid decomposition. A CO methanation reaction also occurred to some extent. The obtained results evidenced that formic acid can be used as a source of hydrogen during catalytic reactions of tar. The change in pressure at 250 and 300 • C occurs because the system needs time to reach a steady state. It should be noted that products of formic acid decomposition might be in equilibrium with each other. The water-gas shift (WGS) reaction described in Equation (3) is an industrial process in which water in the form of steam is mixed with carbon monoxide to obtain hydrogen and carbon dioxide.
The WGS reaction is reversible and exothermic (∆H o = −41.2 kJ/mol). The WGS reaction is thermodynamically favorable at temperatures of 300-400 • C [33]. Iron-based catalysts are typically used industrially. Therefore, the water-gas shift (WGS) reaction also favors the formation of hydrogen and CO 2 .

Effect of Formic Acid on the Catalytic Transformations of Tar
The data on the tar, FA and catalyst loadings, and the selectivity to different fractions are summarized in Table 8. The reaction was performed at 350 • C for 2 h.
In addition to the total tar conversion, it is important to control the composition of liquid products. They were analyzed using the imitation distillation method. The fractions of gasoline (0-180 • C) and diesel (180-360 • C) fractions, vacuum gasoil (360-550 • C), vacuum residue (550-720 • C) and non-eluted residue were calculated according to ASTM D7169 on Agilent 7890B. The fraction compositions of the obtained liquid products are presented in Table 9 and Figure 5.  In addition to the total tar conversion, it is important to control the composition of liquid products. They were analyzed using the imitation distillation method. The fractions of gasoline (0-180 °C) and diesel (180-360 °C) fractions, vacuum gasoil (360-550 °C), vacuum residue (550-720 °C) and non-eluted residue were calculated according to ASTM D7169 on Agilent 7890B. The fraction compositions of the obtained liquid products are presented in Table 9 and Figure 5.  The presented data demonstrate that the addition of formic acid and the catalysts resulted in redistribution of the liquid products between different fractions. The lowest amount of the gasoline (10.8%) and diesel (39.1%) fractions was observed in the liquid products formed by the thermal tar cracking. The addition of formic acid alone or together with the BEA zeolite to tar increased the The presented data demonstrate that the addition of formic acid and the catalysts resulted in redistribution of the liquid products between different fractions. The lowest amount of the gasoline (10.8%) and diesel (39.1%) fractions was observed in the liquid products formed by the thermal tar cracking. The addition of formic acid alone or together with the BEA zeolite to tar increased the contributions of these fractions among the liquid products.
Substitution of the BEA zeolite for the 8% Ni-2.5% Mo/Sibunit catalyst led to a decrease of the gasoline fraction concentration from 16% to 12.7%. This means that the 8% Ni-2.5% Mo/Sibunit catalyst has lower cracking ability than the BEA zeolite. Nickel metal is known to be a good catalyst for methanation and hydrogenolysis of C-C bonds [34][35][36][37]. These properties lead to its higher selectivity to gaseous products (Table 8). Meanwhile, simultaneous addition of the BEA zeolite and the 8% Ni-2.5% Mo/Sibunit catalyst (sample 380) led to the highest yields of both the gasoline and the diesel fractions.
Changes of the sulfur and nitrogen contents in the liquid reaction products were also studied. The results are presented in Table 10. Variations of the sulfur and nitrogen concentrations were symbatic, most likely due to similarities in the mechanisms of desulfurization and denitrogenation of liquid products during the catalytic transformations of tar. The lowest sulfur concentration was observed for sample 5. This result was observed despite the fact that the original carbon support Sibunit contained 0.5 wt.% S. Ni-Mo catalysts are known to be among the best catalysts for purification of diesel fuel from sulfur. When the BEA zeolite is present together with the 8% Ni-2.5% Mo/Sibunit catalyst, it also contributes to desulfurization of the liquid products.

Conclusions
Thermal cracking of tar was studied at 350-500 • C and 1.0 MPa pressure. At 350 • C the tar carbonization resulted in the formation of gaseous and liquid products as well as oil coke. The fraction of liquid products was about 15 wt.%. Only gaseous and solid products were observed when the carbonization temperature was increased to 450 • C. Apparently, the temperature increase led to the conversion of liquid products to gases. The main gaseous tar carbonization products were methane, ethane and propane. H 2 S and COS were observed in the reaction products. Due to this fact, the sulfur concentration on the oil coke decreased in comparison with the precursor tar from 1.5 to 1.28 wt.%.
Specific features of the catalytic tar cracking in the presence of formic acid, BEA zeolite and the 8% Ni-2.5% Mo/Sibunit catalyst were studied at 350 • C 1.0 MPa pressure. The experiments showed that the formic acid addition made it possible to perform efficient hydrocracking of heavy feeds such as tar. Both the tar conversion and selectivity to light (gasoline-diesel) fractions increased in the sequence: tar < (tar -formic acid) < (tar -formic acid -BEA zeolite) < (tar -formic acid -BEA zeolite -8% Ni-2.5% Mo/Sibunit catalyst). Furthermore, significantly lower concentrations of impurities containing sulfur and nitrogen were observed for the (tar -formic acid -BEA zeolite -8% Ni-2.5% Mo/Sibunit catalyst) system. For example, the sulfur and nitrogen concentrations in the tar precursor were 1.50% and 0.86%,